1. Field of the Invention
The present invention relates generally to multiple access technology in a cellular communication system, and in particular, to a method and apparatus for allocating an uplink resource and transmitting/receiving data therethrough.
2. Description of the Related Art
In general, a cellular system supporting 2nd generation (2G) and 3rd generation (3G) mobile communication uses Direct Sequence Code Division Multiple Access (DS-CDMA) technology. The DS-CDMA technology multiplies transmission data by a spreading code, and then spreads the transmission data in a frequency band before transmission.
The DS-CDMA technology is disadvantageous in that it suffers from severe multipath fading. The multipath fading causes interference between adjacent symbols. Rake receivers are used to overcome the multipath fading.
However, the use of Rake receivers causes an increase in complexity of a receiving apparatus. In addition, it is difficult to achieve multiplexing gain for multiple users in a frequency domain using broadband characteristics, and hard to use a high-speed modulation technique due to interference between users.
For these reasons, it is not appropriate to use the DS-CDMA technology in the next generation mobile communication system (4G) in which broadband resources will certainly be used.
Taking the foregoing into consideration, an Orthogonal Frequency Division Multiple Access (OFDMA) technology is attracting attention as a multiple access technology appropriate to achieve high-speed transmission required by the next generation mobile communication system. The OFDMA technology finely divides a broadband frequency into a plurality of narrowband frequencies (or subcarriers), and allocates the subcarriers per user. In this way, the OFDMA technology can increase a length of user symbols, maintaining a data rate.
The interference between adjacent symbols due to multipath fading is relieved by adding a guard time having the same pattern to the user symbols. It is easy to obtain multiplexing gain for multiple users by allocating subcarriers with high channel gain on a per-user basis. In addition, because the frequency resource is finely subdivided, the OFDMA technology is appropriate to obtain resource management gain, such as multiuser gain. For these reasons, various OFDMA-based multiple access technologies have been proposed in recent years.
Typically, Multi-Carrier CDMA (MC-CDMA) and Frequency Hopping OFDMA (FH-OFDMA) technologies are attracting attention as a technology that is superior in relieving interference from an adjacent cell and is appropriate for the cellular environment.
The MC-CDMA technology allocates different codes to different users, and spreads the codes in a frequency band, thereby identifying users. Resource allocation in the MC-CDMA technology is shown in FIG. 1. In FIG. 1, the full frequency band is divided per predetermined sub-frequency (SF) band, and in each sub-frequency band, different codes are allocated to the users. That is, code #1 to code #K are allocated to user #1 to user #K, respectively. Therefore, each of the users spreads transmission data with a code allocated thereto in the corresponding sub-frequency band before transmission.
Therefore, the application of the MC-CDMA technology can relieve interference from an adjacent cell, and scatters the spread chips over the frequency band, thereby achieving frequency multiplexing. However, in the case of an uplink, signals are received at a base station from the users over different channels. Therefore, it is difficult to restore orthogonality between the codes by distinguishing various user signals received through the same subcarrier.
However, the use of the conventional linearized receiver such as a Maximal Ratio Combining (MRC) receiver causes severe performance degradation. This is well disclosed in “Design and Performance of Multicarrier CDMA System in Frequency Selective Fading Channels” submitted to IEEE Transactions on Vehicular Technology by Prasad in 1999.
The FH-OFDMA technology allows a user to continuously avoid fading through frequency hopping, and can obtain frequency multiplexing gain by being combined with channel coding. Resource allocation in the FH-OFDMA technology is shown in FIG. 2. In FIG. 2, a frequency band is divided into a plurality of subcarriers, and the subcarriers are allocated on a per-user basis. The subcarriers are allocated to the users not on a fixed basis, but on a time-varying basis. That is, a set of subcarriers allocated to a particular user is subject to dynamic change according to fading characteristics of a radio transmission line. This is called “dynamic resource allocation” or “frequency hopping.”
The use of the FH-OFDMA technology allows even interference from the adjacent cell to undergo frequency hopping, so that the subcarriers in use do not always suffer from interference. Therefore, the FH-OFDMA technology, if combined with channel coding to level (or equalize) the interference, can relieve the interference from the adjacent cell. However, in the uplink, all users must perform channel estimation every hop, increasing a load caused by pilot signals. In particular, application of a coherent modulation/demodulation technique further increases the pilot load.
In order to overcome these problems, there has been proposed a method of using a non-coherent modulation/demodulation technique after grouping several time-domain symbols. However, the non-coherent modulation/demodulation technique does not use pilot signals, causing a decrease in frequency efficiency.
The foregoing MC-CDMA and FH-OFDMA technologies use code spreading and frequency hopping, respectively, in the cellular system in order to overcome interference between adjacent cells, while obtaining frequency multiplexing gain.
However, if a load between various cells existing in the cellular system is taken into consideration, it can be more effective to use both code spreading and frequency hopping techniques by appropriately adjusting gains thereof according to the user environment, instead of using only one of the code spreading and the frequency hopping techniques. To this end, the multiple access technology for a physical layer needs to define various flexible basic resource units such that they can be appropriately adjusted by an upper layer.
In order to use the coherent modulation/demodulation technique having high frequency efficiency, it is necessary to decrease the load caused by pilot signals, taking into account the necessity to transmit the pilot signals per user. In addition, when the code spreading technique is applied to equalize interference between adjacent cells, there is a need for a multiple access technology capable of minimizing a loss caused by orthogonality damage. In particular, because the orthogonality damage occurs as a serious problem in an uplink environment where user signals are received over different channels together, there is a keen demand for a multiple access technology capable of overcoming this problem.